The nuclear fusion processes power stars.

For stars like our Sun which have internal temperatures less than fifteen million Kelvin, the dominant fusion process is Proton-Proton Fusion.

For more massive stars which can achieve higher temperatures, Carbon Cycle Fusion becomes the dominant mechanism.

For older stars which are collapsing at the center, the temperature can exceed one hundred million Kelvin and initiate the Helium Fusion process called the triple-alpha process.

The Sun fuses about 650,000,000 tons of hydrogen to helium every second, creating heat and electromagnetic radiation. The heat remains in the sun and maintains the fusion reaction. The electromagnetic radiation (visible light, infra-red, and ultra-violet radiation) goes out into space in all directions.

Only a very small fraction of the total radiation produced reaches the Earth. The radiation that does reach the Earth is the indirect source of nearly every type of energy used today. The exceptions are geothermal energy, nuclear fission, and fusion. Even fossil fuels owe their origins to the sun; they were once living plants and animals whose life was dependent upon the sun.

When the universe was very young, immediately after the Big Bang, matter was densely packed and temperatures were high. Fusion in these conditions produced H, He, Li, Be, and B - the first five elements on the Periodic Table.

Once stars that are several times more massive than our Sun reach the red giant phase, their core temperature increases as carbon atoms are formed from the fusion of three helium atoms. Gravity continues to pull carbon atoms together as the temperature increases and additional fusion produces elements from nitrogen to iron.

Fusion in the core of a red giant star ceases when the core is mostly iron. Since it takes more energy to fuse iron than the reaction will produce, energy is no longer being released from the core.

The star begins the final phase of gravitational collapse. The core temperature rises to over 100 billion degrees as gravity crushes the iron atoms together. The repulsive force between the nuclei overcomes the force of gravity, and the core recoils out from the center in an shock wave called a supernova. As this energy encounters material in the star's outer layers, the material is heated, fusing to form heavier elements and radioactive isotopes.

While many of the lighter elements are made by nuclear fusion in the cores of stars, it takes the death of a star to form elements heavier than iron.

 
The nebular hypothesis is one theory about how solar systems form.

Facts about our Solar System.

The Sun produces about 4 X 10²⁶ Watts of energy per second. That's about equal to 9 X 10¹⁰ megatons of TNT per second.

Visible characteristics of the Sun:

• Sunspots
   
• Filaments
   
• Prominence
   
• Mass Ejection